Sulfur oxides are present in many gaseous mixtures, including flue gases from power plants, smelter gases, and other gases emitted from various industrial operations. These gases are generated, for example, in refinery operations, coke processing, and sulfuric acid manufacturing. These sulfur-containing compounds are generally sulfur oxides, i.e., sulfur dioxide and sulfur trioxide.
The contamination of the atmosphere by sulfur oxides, whether present in power plant flue gases, smelter gases, etc., has long been known to be a problem for public health. It adversely affects respiratory systems, harms plant life and corrosively attacks various materials including metals. The development of processes for removing sulfur oxides from gaseous mixtures is therefore highly desirable.
U.S. Pat. No. 3,607,002 to Meyers relates to a process for removing sulfur oxides, e.g., sulfur dioxides, from hot gaseous mixtures by passing the gaseous mixtures over aniline black compounds, e.g., migrosine copper phthalocyanine, pyrolyzed polyacrylonitrile or pyrolyzed polyvinylchloride.
U.S. Pat. No. 4,003,848 to Cotter et al. discloses contacting moist sulfur-containing gaseous mixtures with a crosslinked, water-insoluble polymer of N-glycidyl piperazine or N-glycidyl polyalkylpiperazines at a temperature of about -10.degree. to about 100.degree. C. to adsorb sulfur dioxide from gas mixtures.
It is known to utilize polar, functionalized polymers as ion exchange resins in desulfurization processes. Ion exchange resins typically have ionic groups attached to a polymer, and basically operate by donating or accepting protons. Suitable functionalized polymers for ion exchange processes include, for example, those having polar groups bonded thereto, in particular basic substituent groups, such as primary, secondary, or tertiary amine groups.
For example, U.S. Pat. No. 4,853,191, to Bruening et al., discloses the use of copolymers of Polymerizable vinyl compounds which contain tertiary amino groups as ion exchange resins. These resins are utilized in the presence of H.sub.2 O to remove sulfur oxides from industrial gases.
"Stability and Equilibrium Properties of Macroreticular Resins for Flue Gas Desulfurization", by Chen et al., Ind. Eng. Chem. Res., Vol. 29, No. 3, pp 440-447 (1990), discloses macroreticular ion-exchange resins, based on a copolymer matrix of styrene and divinyl benzene, for use as adsorbents for flue gas desulfurization. One of these macroreticular weak-base ion exchange resin, Dowex MWA-1, was found to be suitable as an adsorbent, in the presence of water vapor in a temperature swing adsorption process, for flue gas desulfurization. Temperature swing adsorption processes typically can take hours to complete a single adsorption cycle.
Unfortunately, functionalized ion exchange resins are very hydrophilic, hence water competes with sulfur dioxide for adsorption. Moreover, water causes swelling of resins, making it impossible to operate in fixed bed adsorbers. For these reasons, the resins have not been used commercially for SO.sub.2 removal.
The current practices for sulfur dioxide removal require wet scrubbing desulfurization techniques. Scrubbers are devices for washing or absorbing gases by dissolving or reacting the gases with liquids, such as, for example, limestone solutions. One common problem with wet scrubbers is the formation of corrosive sulfuric acid aerosols which cause such processes to be difficult to operate and maintain. A further problem occurs in the case of scrubbers utilizing limestone solutions, namely, the formation of CaSO.sub.3 and CaSO.sub.4, which is a pollutant in itself.
In addition, each fixed bed adsorbent material has a certain capacity for adsorbing particular consituents. Consequently, the quicker the cycle times, the higher the efficiency any particular adsorbent bed will be. Unfortunately, many processes in the prior art exhibited long cycle times, i.e., of 30 minutes or more. Thus, there still is a need for an efficient and economical desulfurization process which generates a nonpolluting sulfur end product.